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Show Storm Plot No. 3. Alta 1951 ( Figure 37). This is the life history of a major alpine storm. It was prolonged, 144 hours, during which time there were 64 inches of snowfall and 5.9 inches of water precipitation. It did not become interesting until December 2. Then, for 11 hours, blizzard conditions prevailed with snowfall intensities up to two inches per hour and strong wind action. This is a hazard pattern any trained snow observer would recognize without further assistance. P. I. averaged .11 inch per hour for 11 hours, making a P. I. Factor of 1.20. Settlement was practically zero. However, avalanche action started almost at once with .50 inch of water and 7 inches snowfall, a fact revealed by the timing devices and one which altered the hazard forecast entirely. Sluffing is always a sign of stabilization. The storm now rambled along at moderate levels for two days. Early on the morning of December 5 blizzard conditions set in again with P. I. of .18 inch and S. I. 2.1 inches, but duration was only 5 hours. P. I. Factor for this period was .90 inch. Again the storm tapered off and a second sluff cycle ran during the lull. The storm expended the rest of its heavy ammunition in a 3- hour bombardment the same afternoon, followed by a third sluff cycle. This storm deposited an impressive amount of snow, reached blizzard proportions on three different occasions- and accomplished nothing so far as dangerous avalanche action is concerned. It was a classic exercise in hazard forecasting. Hazard approached critical levels twice. On December 2 the early sluff cycle canceled out the other factors. The situation was much more ominous on the 5th. Here the blizzard pulled its punch just in time. To focus on the P. I. curve we find that high P. I. and S. I. coincide, logically enough. We also see that it is possible to have high S. I. and low P. I. In this case the reversal coincides with a drop in windforce. Before we run too far with that observation, we should note that a high- level P. I. persisted- into a low windforce period. To conclude, it is difficult for an observer without mechanical aids to detect minor shifts in S. I. and windforce which are significant in a borderline hazard situation such as this. Unaided, he cannot detect at all the sluff cycles or the important variations in P. I. and settlement. While the average density of the snow of this storm was .09, it is obvious from the P. I.- S. I. relationship that densities during the storm varied widely. Storm Plot No. 5. Alta. 1951 ( Figure 38). By way of contrast to No. 3> this storm lasted 29 hours, total snowfall was 22 inches, and total water precipitation was 2.50 inches. Blizzard conditions prevailed for 10 hours. There was never much doubt about its intentions. The snow type was bad: granular and pellet transition of unusually high density against the crystalline snow of normal density for No. 3. Windforce was high and during the major portion of the storm settlement was almost zero. The S. I. and P. I. indicators were red. There was no sluffing. - 104 - |
